Wave trap enabling simultaneous receiving and transmitting from same antenna



Dec. 26, 1961 D. s. WILLARD 3,015,099

WAVE TRAP ENABLING SIMULTANEOUS RECEIVING AND TRANSMITTING FROM SAME ANTENNA Filed Oct. 21, 1959 ls: RF STAGE AVC INVENTOR. DAVID S. WILL D BY Wm- ATT ymbg lii/ United States Patent Ofifice 3,fil5,099 Patented Dec. 26, 1961 3,tll5,il99 WAVE TRAP ENABLING SEMULTANEQUS RE (IEEVWG AND TRANSMTTTHNG FRQM SAME ANTENNA David S. Willard, High Rolls, N. Mex., assignor to the United States of America as represented by the Secretary of the Air Force Fiietl (let. 21, 1959, Ser. No. 847,351 1 Claim. (Cl. 343-180) (Granted under Title 35, US. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the United States Government for governmental purposes Without payment to me of any royalty thereon.

The purpose of this invention is to provide a circuit for coupling the output of a transmitter and the input of a receiver to a common antenna for simultaneous transmitting and receiving at different but not widely separated frequencies without undue loss of transmitter power or overloading of the receiver input.

The details of the invention will be explained with reference to the accompanying drawing which shows a schematic diagram of the coupling circuit.

Transmitter 1, operating at a frequency f is coupled to the antenna 2 through transformer 3. Receiver 4, operating at a frequency f which differs from f has the grid of its first R.F. stage coupled to the antenna 2 by means of a circuit comprising a series resonant circuit L -C tuned to y,, a shunt condenser C a parallel resonant circuit L -C tuned to f and a series resonant circuit L C tuned to f Condenser C serves as a by-pass condenser around the AVG circuit of the receiver which is connected to the grid of the first RF. stage through L Since the resonant frequency f of circuit Is -C is f this series resonant circuit offers negligible impedance to the transmitter frequency and substantially all of the transmitter output voltage at the secondary of transformer 3 is applied between the antenna and ground as desired for maximum radiation. The purpose of circuit L C condenser C and circuit L C is to reduce this voltage enough before it reaches the receiver input to prevent overloading of the receiver. As seen from the drawing, the antenna voltage during transmission is also applied across the series circuit consisting of parallel resonant circuit L --C and condenser C Since the L -C circuit is tuned to f it presents a high impedance to currents of this frequency. The magnitude of this impedance equals 21rf L Q, where Q=21rf L /R R being the radio frequency resistance of L at f,;, and may be made very large by the use of an inductance L of high Q. Condenser C is made large enough to have an impedance at I that is very low relative to the impedance of L C at this frequency. Therefore, a very small fraction of the voltage of frequency f appearing between the antenna and ground appears across C The radio frequency voltage across C is also applied across series circuit L -C Condenser C is a relatively large by-pass condenser so that the lower end of L is eifectively connected to ground for radio frequencies. The resonant frequency f of circuit L C is made equal to the receiving frequency f With sufficient separation of f and f the f voltage across L may be reduced to a magnitude comparable to the f signal magnitude across C When f is lower than f the position of L and C should be as shown in the drawing since the greater f voltage appears across C For an f greater than f, the positions of C and L should be interchanged since the greater f voltage will then be developed. across L The operation of the circuit coupling the antenna to the receiver input will now be considered for the receiving frequency f Since the circuit L C is tuned to f it has a relatively high impedance at This prevents the relatively low output impedance of the transmitter from loading the antenna at f,,. The receiving frequency antenna voltage, in the same manner as the transmitting frequency voltage, is applied across the series circuit consisting of circuit L -C and C However, circuit L -C which is tuned to f has a relatively low impedance at 7, so that a greater portion of the antenna f voltage is developed across C; than was the case for the f voltage. Further, circuit L -C which is tuned to series resonance at f develops voltages across L and C that are far greater than the voltage across C being Q times this voltage. Therefore, if an inductance L of very high Q is used, the f voltage developed across L and applied to the input of the receiver will be many times the voltage of this frequency across C From the foregoing it is seen that high attenuation of voltages of frequency i occurs between the antenna and the receiver input. This, together with the fact that the receiver is tuned to receive 7, and reject f prevents the transmitter from overloading the receiver. On the other hand, the 7, signal is conveyed from the antenna to the receiver input with little or no attenuation, the loss introduced by L -C and C being made up by the voltage amplification produced by series resonant circuit L -C I claim:

A circuit for coupling an antenna to the output circuit of a transmitter and the input circuit of a receiver, comprising: a transformer having a primary winding coupled to the output circuit of said transmitter and a secondary winding; a first series inductance-capacitance circuit resonant at a predetermined transmitting frequency; means connecting one terminal of said secondary winding to said antenna and the other terminal through said first series inductance-capacitance circuit to a point of reference potential; a shunt capacitor having one terminal connected to said point of reference potential; a parallel inductance-capacitance circuit resonant at said transmitting frequency connected between said antenna and the other terminal of said shunt capacitor; a second series inductance-capacitance circuit resonant at a predetermined receiving frequency diiferent from said transmitting frequency connected in parallel to said shunt capacitor; and means coupling the input of said receiver across one of the elements of said second series inductancecapacitance circuit.

References Cited in the file of this patent UNITED STATES PATENTS 1,188,531 Carson June 27, 1916 

